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converting linear BBQ dish to circular polarization



A few days ago I asked:

I would like to take a survey of those who have converted a BBQ dish/dipole
feed to circular polarization.

1. How much AO40 receive improvement did you get?
2. What kind of feed did you use?
3. How would you judge the difficulty/durability/repeatability of the
conversion?


Here's a summary of the replies I received describing conversions to CP:

Howard G6LVB reported a slight improvement converting a TSI dipole feed to a
helix with his umbrella dish.  The dish was never linear polarized.

Greg KO6TH sent a link to his web site
http://home.jps.net/~gregd/hamsite.htm
<http://home.jps.net/~gregd/hamsite.htm> 
He couldn't really quantify if his conversion resulted in much improvement
because his operating skills and the off-pointing angle improved over the
same time period that he made the modifications.

K5GNA informed me that Jerry K5OE converted a TranSystem dish and
downconverter to circular polarization.  That's what I have, so I was very
interested.  Jerry was kind enough to send me a link to his web site which
has a thorough explanation and good diagrams and pictures.  He approached
the design, testing, and implementation VERY methodically and presented the
results at the 2001 AMSAT-NA Symposium.

You can read about Jerry's project by going to http://members.aol.com/k5oe
<http://members.aol.com/k5oe>  and clicking on the MMDS link.  He never
compared it to the "stock" setup, but on the air reports indicate that he
hears AO40 about 2 S-units better than people who use the stock TranSystem
setup.  That improvement is primarily due to three things:
1. Conversion to circular polarization
2. Optimized dish illumination with a 6.25 turn helix and cup reflector
3. Addition of a DEM preamp to lower the noise figure

Jerry cut his dish down to 26x26 inches.  He hears better with a SMALLER
dish!  That minimizes my concern about additional wind and ice loading when
adding mesh to the dish.  Jerry believes that his documentation makes the
design easily reproducible. Overall, Jerry's conversion addresses all three
of my concerns, but I would consider adding a radome over the helix to make
it more immune to rain and snow.  I now have one more thing to add to my
winter project list.

Wayne Estes W9AE
Beijing, China (until 03 November)
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